Task limits are the upper limit of the number of cards you can request to work on at a given time. For example if your task limit is 100, you could request a job of 50 cards and while working on it request up to 50 further cards.
Once you have reached the upper limit of your task limit, if you request a job that is available a message will display that this would take you over your task limit and you will not be able to request it. Similarly if a job is available that has a higher quantity than your task limit you will not be able to request it.
Task limits can be impacted by things like submitting work late, a job with many errors or that didn't meet quality standards or job cancellations. A lower limit means you will only be able to request a job quantity up to that limit but will allow you to continue to request and work on jobs and prevent your scribe account being closed.