Plugins are small programs that do a specific task. That plugin can be added, or not, to Sigil if you need that task accomplished. If you find a plugin that you want to add, simply download the zip file (leave it as a zip and don’t change the name) somewhere convenient like your desktop. Then, in Sigil, select Plugins/Manage Plugins from the menu. Then Add Plugin and point it to the zip file. You can assign up to 10 of your favorite plugins to display as a button in the toolbar, the rest can be accessed by the plugin menu.
The description of each Plugin, on its thread here in MR, should explain how to use it.
Your question about changing </span><p> can be done using the find/replace function, and possibly something called regular expressions (regex), but you will need to find something that more uniquely identifies the problem area…narrow down the search so you don’t get 4000 hits.
I don’t quite understand what you are looking for re: page break. Can you explain a little more?