suck at: bad at doing something
Situation #1: Two coworkers
Bill: Can you help me with this spreadsheet? I can't figure out the right formula
Linda: Probably not. I suck at Excel.
Bill: Could you take a look at it?
Linda: Sure... Yeah, I don't know what I'm looking at. Have you asked Karla? She's great at Excel.
Bill: No. I'll ask her. Thanks.
Explanation: Bill was having trouble with a formula in Excel and asked for Linda's help, but she said that she sucks at Excel, which means she is not good in that software.
Situation #2: A group of friends is playing basketball and another one joins.
Todd: Hey, Greg! Come join us!
Greg: No, that's ok. I'll just watch.
Brian: No! Join us! Todd's team has one more player than us, so you will make us even.
Greg: You don't want me on your team. I suck at basketball.
Brian: You couldn't make us any worse! We're already down by 10 points.
Todd: Great! Now Brian can't keep saying that he's losing because of uneven numbers!
Explanation: Todd and Brian are playing basketball (on different teams), and they both ask Greg to join them. Greg says that he sucks at basketball, but Brian convinces him by pointing out that his team is one player short, and that his team is already losing anyway.