“Keep in mind that it's a lot warmer there at this time of year.”
Your roommate is going on a trip for a few weeks to Bali, which she's never visited before. You've been to Bali before, and want to give her advice on what to pack for the trip. You say this because you don't think she needs to take a lot of heavy clothes.
Keep in mind that it's a lot warmer there at this time of year.
"Keep in mind" means to remember. But the word "remember" can be used in a lot of different kinds of situations. "Keep in mind" is more specific. It means to remember a fact and continue to think about it while you're making decisions.
For example, if you're planning an event for a club you belong to, you can tell the people that are helping you:
Keep in mind, our budget is pretty small.
In this case, everyone knows what the budget is, but you want them to remember that when they make suggestions for the event.
You can include "that" in this phrase or leave it out in more casual speech:
Keep in mind that you'll need to submit your application no later than Monday the 31st.
Keep in mind it's due on Monday.
Use this phrase when you're talking about things that usually happen during a certain season. It can be used when talking about weather, business, holidays, and other topics. For example:
The stores are always packed at that time of year.
It tends to rain a lot at this time of year.
Notice that the phrase is "that time of year", not "a year" or "the year".
it's (comparative adjective) (somewhere) than it is (somewhere)
Use this phrase to compare descriptions of two places. Some more examples:
It's less crowded there than it is in New York.
It's noisier in Calcutta than it is in Singapore.