I came across this line when i was as reading a tutorial

i.e.
compiler increases the compilation speed by performing in-memory compilation....what does this statemant mean??.....and what is the normal way of compilation

It may mean that the compiler does the severals passes without writing on the disk :

For example, gcc (without the -pipe option) writes a temporary file on the disk between each compilation step.

the gcc suite is made of many tools that communicate by default using temporary files. The -pipe options make them communicate using pipes, which is more fast.

I don't know if it is what you meant by 'in-memory compilation'