What is the flag in the picture?

Rasta Fist Barbed Wire Flag

It appears to be a "Black Power" statement.

The raised, clenched fist has been a symbol of the Black Power movement for decades and most-recently featured prominently in Black Lives Matter banners and signs. The red, yellow and green bands are also popular in statements of Black Nationalism.

The color combination is based on the flag of Ethiopia, often seen as an African nation that successfully resisted European colonization.  After they expelled the Italians in the late 19th Century, the Ethiopian colors were unofficially adopted as a symbol of resistance against colonial (i.e. white) oppression across Africa.  This is why the colors feature prominently in the flags of so many African nations today.

The symbolism was carried to the Americas via the black populations in Caribbean nations, which is why they also feature so prominently in "Reggae" culture.  The addition of black elements, or the replacement of the yellow band with a solid black is often seen as a statement of Black Nationalism as opposed to "African" nationalism.