SAS stands for "side-angle-side".
There are two triangles, ABC and DEF. We are given that side AB is congruent to side DE, angle A is congruent to angle D, and side AC is congruent to side DF. Notice that the congruent angles are included between the congruent sides. This is very important.
When these conditions are met, the two triangles must be congruent, which means their other parts are congruent. That is, angle B is congruent to angle E, side BC is congruent to side EF, and angle C is congruent to angle F.
This is taken as one of the fundamental postulates of geometry. The reason it is a "common-sense" notion can be seen by looking at the construction of two triangles that meet the SAS condition.