When something is up for grabs, it is available; it can be obtained freely without stealing from someone else.
This is often used in electoral politics, but has other applications.
Example: (Context: United States) "In the mid-term Congressional election this November, all House seats and one third of Senate seats are up for grabs."
Barring special elections, which can occur as a result of deaths of members serving in office, all House seats and one third of Senate seats are up for grabs every two years, but American news articles will write as in the above example. A reader unfamiliar with the American political system might think that this situation is somehow exceptional.
House members serve for two year terms; Senate members serve for six, and their elections are staggered so that a third of all seats are subject to election every two years (so that one vote is held for both houses of Congress). We call these elections "mid-term" because they occur in the middle of the Presidential term of office (four years).