The spleen is an internal organ that lies under the diaphragm on the left of the abdominal cavity. Sources refer to it being rubbery and red, but I wouldn't know about that, having never seen or felt one first hand. It's one of those organs that was originally thought to be useless, but I'd take such judgements with the proverbial grain of salt, for "the current thinking is" that the spleen has a more important function in fighting against infection than was previously realized.
The spleen is made up of lymphoid tissue, as are the lymph nodes, tonsils, and thymus. The spleen's functions are to produce lymphocytes (an important part of the immune system) and filter out and destroy foreign organisms (bacteria, parasites, and debris) and old red blood cells from the bloodstream. During fetal life, the spleen also produces red blood cells, but after birth, the bone marrow takes over this duty. However, if the bone marrow can no longer do its job, the spleen will revert to its former function. The spleen has another useful function: it's a kind of blood reservoir, and when additional blood is needed, the spleen contracts, forcing the stored blood into circulation. Liver disease may cause the spleen to become enlarged, necessitating removal; it may also be extracted in cases of physical trauma. Otherwise, it seems a good idea to leave it there, just in case.
You may have also heard spleen used in a metaphoric sense, as in someone venting their spleen. The spleen was once thought of as the seat of emotions like ill humour, peevishness, and spite. So, venting one's spleen means being crabby, bad-tempered, and nasty. You can refer to such unpleasant people as spleeny, spleenful, splenetic, or splenetical, if you wish.