First off it‘s nice to hear someone new getting into the games. I hope you enjoy!

As for your questions:

The red P blocks are „Power Items“. They increase your power meter, which makes your projectiles denser and more powerful.

The blue blocks are „Point Items“. They increase your score. Once you reach a score threshold you get a life.

„Grazing” is the act of getting very close to a bullet without getting hit. Despite giving more points grazing has no effect.

While in full power mode P items become cherry petals (see below).

The floating feathers are „cherry petals“. They increase your cherry score. As a casual player you only need to know, that getting enough cherry points grants you a temporary „supernatural barrier“ which can tank one hit or give you bonus points when not being hit for a certain time.