I've never done a game jam before and I kinda want this to be my first one! Just doing research first before I join!~

But I'm wondering, when it's over, can I expand on the game I've made in the jam? Like if I left the game on an cliffhanger and want to continue making it into an series?

Absolutely! Some great games (some are considered legends like Last of Us) came from game jams! 
Entering a jam does not revoke any ownership of your content. It's your content and whomever you work with if you work with anyone. 

Hello Katie E, you are 100% allowed to expand on your game that you will be making for the jam. Game jams are just a helping hand to give ideas to someone so they can create a game. For example Oneshot a rpg maker game was created for a jam and was later expand on and is a commercial game. Also like Pixality said it's your content! So you have the final say of what you are going to do with it.

^^b I do hope you have a fun time in this jam btw.

Thank you for the answers! I didn't know much about game jams so it's good to know! I'll definitely join this one! Have a nice day everyone!