Inside of the leaves are seeds I think (in that lighter coloured area you can see on each leaf). Flowers are small and white.

The flat seed pods reminded me of Honesty Plant (Lunaria), but the flowers of that are usually purple. It could be Stinkweed (Thlaspi arvense), which does have little white flowers.

How big are those seed pods? How tall is the plant? How many petals do the flowers have? Are the leaves stinky when crushed?

Yes, Thlaspi arvense. Growing in a bit of shade, I see....

We call it Penny Cress.