Hello.
Short run and long run are the theoretical concept of economics which can be studied with a curve and simple explainatory definition of short run and long run are given below:

• SHORT RUN:

In this one factor of production (i.e, Capital)is fixed this is a time period which is fewer than four-six months.

• LONG RUN:

In this all factors of production of a firm are variable (e.g. a firm can build a bigger factory) this is a time period which is greater than four-six months/one year.

I hope you understood.
Best of luck.