There's more to halal than food
Often, when someone talks about "halal" they are usually talking about food, specifically about meat prepared using Islamic methods, but there is more to halal than just food.
The word halal means permissible and it is the antonym of haram, forbidden. The word haram is used to label all sorts of sinful things, but the word halal is often just used for food. It shouldn't be like that. If the word halal means permissible, it should be attributed to many things.
There are far more things in Islam that are halal (permissible) than there are haram (forbidden). The general law of Islam is that if there is no proof of something being harmful or described as forbidden in the Quran or forbidden according to the teachings from the Prophet Muhammed (SAW), then it is halal by default. Islam is a positive religion and promotes the doing of righteous and beneficial deeds and God has made many things halal for us and we should be grateful. Allah says in the Quran:
[5.87] O you who believe! do not forbid (yourselves) the good things which Allah has made lawful for you and do not exceed the limits; surely Allah does not love those who exceed the limits.
[5.88] And eat of the lawful and good (things) that Allah has given you, and be careful of (your duty to) Allah, in Whom you believe.
It is unfortunate that a lot of Muslims culturally care more about their food being halal than their actions. You find a lot of Muslims will not eat haram food but they will commit many sins and this makes no sense. A Muslim is not only meant to follow dietary laws but must also follow the other laws, be a good person and be kind to their neighbors. If Muslims paid as much attention to the other laws as they did the dietary laws the world would be a better place.
Anyway, let's get back to the question of food, albeit let's skip the discussion about ritual slaughter (dhabihah). There is more to halal meat than its' being correctly slaughtered according to Islamic law -
For example, you cannot call what you eat halal if it contains or has been contaminated with haram things. When haram and halal substances are mixed together the result is a haram mixture, much like when salt water is mixed with drinking water, the result is salty water, not recommended for drinking. So always check the ingredients of what you are eating.
Haram foods are often also ritually impure, najis, so you have to be careful they don't come into contact with halal foods. You also need to make sure that utensils and the hands preparing your food are not passing this impurity (najasah) onto your food through moist-contact and contaminating your halal food making it haram. For example, a fork used to pick up pork should not be used to handle halal meat.
Then there are things that go beyond the actual food. You have to consider the origin, the country, company or source of the food - Is the food being given grown on stolen land that is still being claimed, like in the case of products from Israel (Occupied Palestine)? If it is then it is haram. How about the people you are buying from - did they obtain their merchandise legally and fairly? If they are selling stolen or illegal products then it is forbidden to buy from them. And another question you need to ask: Is the money going in to the company from a permissible source? If the person you are buying from is a criminal or primarily invests in sin and corruption then you are not permitted to fund them by buying their products.
It is your Islamic responsibility to check where your food comes from and how it is prepared. But there is one more important thing you need to consider -
If the money you use to buy halal food or other products is from a haram source, be it stolen, defrauded, illegal, laundered, or any other type of ill-gotten money, then whatever you have bought with it becomes haram because you have effectively stolen the item through using stolen money to pay for it. While technically the item itself might be permissible, the fact that it was bought with dirty money makes it dirty as well and you are committing a sin just by consuming or using things obtained with dirty money.
So be mindful and remember - there's more to halal than just food.