Tolerances can be defined as the permitted variation from a given dimension or quantity. This variation can usually be in either of two directions and so can be regarded as positive or negative.
The immediate reason for specifying tolerances is to establish the ease of construction without the necessity of later modifying parts to fit together.
A more long-range reason is to ensure that the structure will perform as needed, particularly with respect to safety. The parts of the structure should have adequate strength and be of such shape and dimensions that there is no danger of structural failure through movement of the parts.
Joints should be sufficiently close to the design to perform properly and, where necessary, be capable of being sealed against intrusion of water, wind, dirt and debris. Tolerances on the location of reinforcing steel should be adequate to ensure the place-ability of concrete all around the reinforcing cage.
Usually the owner or specifier prefers to go with the tolerances which are standardized. Keeping this in mind, the most favored Tolerances are according to the ACI 117.
The video here, gives the allowable tolerances for different members of the structure. The tolerances for concrete reinforcements, slopes, dimensions, location, spacing, columns and beam dimensions, bar bends and ends, etc. all are specified and explained in the video tutorial.