Concrete slabs may break at the joint and either 1) settle/sink or 2) lift. Settling is generally caused by water getting into the sub-grade, which creates an unstable base for the concrete and causes it to sink down. Lifting is generally caused by tree roots pushing the concrete up. Uneven concrete slabs can be a trip hazard, so they should be fixed as soon as possible by a professional concrete contractor.
The best remedy for either situation is to remove and replace the concrete, making sure that the cause of the problem is addressed. In the case of settling, the sub-grade will need to be replaced and properly compacted. Tree roots may be clipped, or contained with root barriers.
Alternatively, uneven concrete slabs can also be grinded down, to create an even surface. This will not address the outlying cause of the problem, and will therefore only be a temporary solution.