what does this part of the .css mean? a img vs p img

hi there,
i’m working on my theme, and i’m not sure what the difference between the ‘a img’ and ‘p img’ is.
basically, when i insert images into a post, i want them to have a border around them (which i did) — but when images are in the sidebar, no border.
i took off the border in ‘a img’ and it removed the border from one of my sidebar image and not the post image. then when i removed the border from ‘p img’, it took off the border from both the post and another sidebar image.

i’m also working locally on wordpress so my actual page doesn’t have the theme up yet. but the theme is “untheme two sidebar”

a img {
border-style: solid; border-width: 0px; border-color: #6979a2;
}

p img {
padding: 3px;
border-style: solid; border-width: 0px; border-color: #6979a2;
max-width: 100%;
}