to prevent a populist president from becoming a virtual tyrant.
The 2 term limit was imposed after FDR, who was wildly popular with the people, had served for 16 years and whose reign was only ended by his death. The politicians of all political sides of the aisle thought that FDR was horrible, but they could not get rid of him due to his popularity with the people. So after his death, they voted in the term limit to ensure for future generations that it could never happen again.
During FDR's reign we also saw how a president who was in office for longer than 2 terms could successfully upset the "checks and balances" of the system by eventually being able to put in enough patsy judges to vote for anything he wanted. There is a lot more to the consequences of FDR's long reign as well, but that is one example to get your started.
no it is not 2 terms in a row. the 22nd amendment states that a president can only be elected twice and does not need to be consecutive, or once if he has served more than 2 years due to the death of the previous president (ie he was VP, and the prez was shot during his 1st year)