Is forgiving someone an issue in Buddhism and what does it mean? In most religions forgiving each other or by God is very common but is in Buddhism forgiving something wiht any value? If so what does it really mean, what does it effect and how is it done?
Hatred is, indeed, never appeased by hatred in this world. It is appeased only by loving-kindness. This is an ancient law.

Source: Dhammapada Verse 5 Kalayakkhini Vatthu
In Buddhism forgiveness is between one another than by an external agency. Having said this no external agency is behind Karma. It is only your tainted or pure mind. And once done the results follow.

All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with an evil mind, 'dukkha' follows him just as the wheel follows the hoofprint of the ox that draws the cart.

Source: Dhammapada Verse 1 Cakkhupalatthera Vatthu
... similarly for good deeds good results follow ...

All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with a pure mind, happiness (sukha) follows him like a shadow that never leaves him.

Source: Dhammapada Verse 2 Matthakundali Vatthu
Following highlights no external agency can affect Karma than the mind itself:

This is explained in the Pathama Mahanama Sutta, by the simile of an earthern pot filled partly with gravels and stones and partly with fat and butter. By throwing this pot into water and smashing it with a stick, it will be seen that gravels and stones quickly sink to the bottom while fat and butter rise to the surface of the water. Likewise, when a person who has established himself in the five wholesome dhammas of faith, conduct, learning, charity and insight dies, his body remains to get decomposed but his extremely purified mental continuum continues in higher states of existence as birth-linking consciousness, patisandhi citta.
In the concluding suttas are expositions on the Middle Path, the Four Noble Truths and the Noble Path of Eight Constituents.

Source: Guide to Tipitaka - 6. Samyutta Nikaya by Professor Ko Lay