This is a poppy - a Papaver. Note the four petals and in the center of the flowers the characteristic thick ovaries with the crown-shaped top. Later, the dried capsule will open at the crown and release the seeds. The leaf rosette on the ground and the hairy stems are also characteristic.
Depending on where you live, you may be more familiar with the common red poppy, but the family has members in various shades of reds, oranges, yellow, white and lilac tones. And of course gardeners have been working on creating lots of variations as well.
Poppies are also grown for their seed: Papaver somniferum, the bread poppy, and some of these also for the alkaloids in their latex sap, which will make opium.
Whether you are looking at a weed or not depends on your personal taste - the common poppy was often considered an unwanted dweller in corn fields (until modern agriculture and herbicides put it on the list of endangered plants), on the other hand, gardeners have always cultivated poppies for their fragile, yet bright flowers. As always: one gardeners weed is another’s prized possession. If you decide you don’t want it, it’s rather easy to pull up and if you like it, harvesting seeds and sowing them wherever you like is also easy.