Partially means part way, not complete. If you’ve partially completed a test when the bell rings, you better work faster to finish up next time!
A part is a portion of something, not the whole thing. Similarly, the word partially refers to a part of something only. A partially brick house is not all brick. Anything that is partially true is only true to some degree, like if you tell someone you’re in a band, but really you just carry the guitars. This word means about the same as partly, and the opposite word would be wholly or entirely.