what is idgham mathalia?

Assalaam alaikum wa rahmatullah,
Most likely you are referring to idghaam mutamathilayn, also called idhgaam mithlayn, translated into English this means the merging of two of the same type of letters into each other.  Please see the following links to lesson on idghaam for further details.

http://www.abouttajweed.com/the-two-same-similar-close-and-far/the-two-the-same-the-two-similar-the-two-close-and-the-two-far-p-3.html

http://www.abouttajweed.com/the-two-same-similar-close-and-far/the-two-the-same-the-two-similar-the-two-close-and-the-two-far-p-2.html

Wa assalaam