A “factor” is something that affects something else. It’s very similar to “reason”.For example:

Those cold winters were definitely a factor in my decision to leave Toronto.

Or imagine that you are talking to a friend about the political protests in your country’s capital. You tell your friend:

Corruption is a big factor.

You can use “factor” like this in formal or casual speech, but we often use it to discuss complex ideas. If a decision or debate is very complicated, you can say:

There are a lot of factors.