The amount of data that you have included in your plan (data allowance) depends upon what plan you've signed up for. There are several ways that carriers enable data allowances.
You would need to check the specific allowances for the plan that you are on - simple enough to do by checking your bill or visiting the carriers website to check.
Data allowances with a cap - say that you are on a 5GB monthly plan. Some carriers have a fixed amount of data that you can use, once you hit this threshold of usage you wouldn't be able to access the internet or use apps requiring a data connection - until the following month. You can visit your carriers website to check whether your allowance is capped or not by searching for the plan that you are signed up to.
Data allowances at full speed, then slowed - this is a plan that gives you a specific amount of data at full speed (4G LTE for example). Once you have used the first 5GB you will still be able to access data but it will be at a slower speed. Some carriers slow the speed to 3G, some to 2G (which is basically unusable for any type of streaming). Check your allowances with your carrier.
They use terminology like "first 5GB at full speed then slowed"
Unlimited data allowance - be very careful before signing up to an unlimited data plan. There are genuine unlimited plans where all the data you use is at full speed 4G LTE and then there are other plans that are unlimited but they have a maximum amount of data delivered at full speed before its slowed. For example, unlimited data with 7GB at full speed, then unlimited data at 3G speeds
Heres a selection of plans and carriers that provide full speed unlimited data plans
Here are a selection plans that have unlimited data plans at full speed then slowed