Hi Sami,
I think Clemens have explained in detail about field symbols very nicely.
Apart from this fieldsymbols in abap are like pointers.We prefer as it is performance wise faster.
After declaring fieldsymbols we have assign a pointer or u can say point to that fieldsymbol so as to use later.
Use of ( P_TAB) in select query is example of Dynamic select statement.
P_TAB value is filled at run time. And it is written in parenthesis so as to hold the content at runtime.
Hope u r clear now.
Regards,
Dipti