The "coladera" can be a dome-type strainer put over roof drains, or can be a flat mesh strainer. It is used to keep large objects from obstructing water flow.

You can see some examples from this Spanish (Mexico) product website:

http://www.helvex.com.mx

Here is an English definition of what a "roof drain strainer" is:

Roof Drain Strainer
A wire mesh strainer used to prevent leaves, twigs, and other debris from entering and clogging a drainpipe.
http://www.peakrealtors.com/glossary/r10.htm