The AVP stands for Attribute Value Pair. If you are familiar with any programming language then it is like a variable, which can be assigned any String or Integer value, you can later use that variable in your script (kamailio.cfg) or pass it to any module function that accepts AVP in its input arguments. e.g.
$avp(myuser) = "someone"; # AVP declaration with simple string value assignment.
$avp(myuri) = "sip:" + $avp(myuser) + "@
somedomain.com"; # AVP value assignment may include other AVPs.
m_store($avp(myuri)); # The value of AVP will be passed to module function m_store.
There are several modules which allow you to declare an AVP in module parameters section. You can then assign value to that AVP in your script, and any function call after that assignment will allow module's internal code to use that AVP value and/or store the result of function in that AVP. e.g.
modparam("msilo", "extra_hdrs_avp", "$avp(myhdrs)") # AVP declaration
...
$avp(myhdrs) = "P-Network-Info: 3G/LTE\r\n"; # Assign value to AVP
m_store($tu); # m_store, will check if you have assigned any value to declared AVP and uses it.
Hope this helps.
Thank you.