Question - What do you mean by WSP?
Answer -
WSP stands for Web Solution Package. It is a cabinet file that contains assemblies, resource files, features, images, application pages, site definitions etc. into single file.
Whenever any customization or upgradation or updation is carried out in SharePoint, various items like assemblies, resource files, features, images, application pages, site definitions etc. are required to deployed.
It becomes difficult to deploy these items separately in development, staging and production. This called for a powerful deployment tool in the form of WSP.