You have a sequence, this means an ordered list of numbers
Let us call the first U1, the second U2, the third U3, the fourth U4 and so one. The Nth one will be UN.
Here you have: U1=7 U2=11 U3=15 U4=19
And so more generally: UN-UN-1= 4
which can be written: UN=UN-1+ 4
And this is the formula that generates the sequence. This kind of sequence has an important role in mathematics and in practical life.