Substitution means replacing a variable with a number or another expression. If y=3x+2 and x=5, you substitute 5 for x to get y=3(5)+2=17.
This is how you verify solutions. If you think x=4 solves 2x+1=9, substitute and check: 2(4)+1=8+1=9. It works.
In algorithm analysis, you substitute input size n into time complexity formulas to estimate runtime. If T(n)=2n2+3n, then T(100)=20000+300=20300 operations.