what does it mean calculate multiple lines?

I have a class that contains multiple recordset and in every recordset i have a field named credit. I want to add a field sum which will calculate all the credits.

maybe you can made a "sum" field computed one?

https://www.odoo.com/nl_NL/forum/help-1/question/computed-field-that-sums-records-72650

also in treeview you can add to any field a sum property, like:

<tree>
    . . .
    <field name="field_name" sum="Sum Popup Text" />
    . . .
</tree>

The later option will add sum at the end of the corresponding column in your list.

Here is my view: where am i going to add the sum?? and sum means what is it a dynamic function or i have to define it??

I do not see your view... you can edit your question and add the view you want to post there. in secod case sum property in xml do not require you to implement anything, it's existing feature, just add some pupup text for it in xml and that's it. But in first case, if you choose to implement computed field, then you'll have to implement it.

Abdel,you must undergo training before asking basic questions. In the Odoo, the official modules are very rich to answer your questions. http://doc.odoo.com is content rich too. You may contact us on contact@serpentcs.com and we will snd you the relevant materals.