Crashed Probes are found on Venus in craters. They are rare, you may have to travel far and wide to find one.
They represent all that remains of earlier probes sent from the Overworld to investigate Venus. Space probes do not survive for long in the highly corrosive atmosphere of Venus.
The Crashed Probe block has some contents which can be obtained by right-clicking it to open it, similar to a chest or machine block. Normally it contains many metal ingots and some compressed metals - so it has high value.
The Crashed Probe block itself cannot normally be obtained (except in Creative Mode). If a Crashed Probe block is somehow obtained and then placed down by the player, it will be empty - it will not contain metals or any Radioisotope Core.
As with other valuable blocks in the game, you can create a block which looks like a Crashed Probe, by shift-clicking a Lighting Panel block against it to pick up its look. Then any Lighting Panel blocks you place will have the appearance of a Crashed Probe. (In fact they will be lighting blocks, not Crashed Probes.)