From documentation:

This field priority is available in the view definition, and is 16 by default. By default, OpenERP will display a model using the view with the highest priority (the smallest number). For example, imagine we have two views for a simple model. The model client with two fields : firstname and lastname. We will define two views, one which shows the firstname first, and the other one which shows the lastname first.

<!--

Here is the first view for the model 'client'.

We don't specify a priority field, which means

by default 16.

-->

<record model="ir.ui.view" id="client_form_view_1">

<field name="name">client.form.view1</field>

<field name="model">client</field>

<field name="type">form</fiel>

<field name="arch" type="xml">

<field name="firstname"/>

<field name="lastname"/>

</field>

</record>

<!--

A second view, which show fields in an other order.

We specify a priority of 15.

-->

<record model="ir.ui.view" id="client_form_view_2">

<field name="name">client.form.view2</field>

<field name="model">client</field>

<field name="priority" eval="15"/>

<field name="type">form</fiel>

<field name="arch" type="xml">

<field name="lastname"/>

<field name="firstname"/>

</field>

</record>

Now, each time OpenERP will have to show a form view for our object client, it will have the choice between two views. It will always use the second one, because it has a higher priority ! Unless you tell it to use the first one !