Electron density is more or less an idea of how many electrons are in a given region.
Electronegative elements draw electrons close to themselves and tend to be electron-dense, whilst less electronegative elements tend not to be.
In the case of aldehydes and ketones, they have a C=O bond in their structure, in which the oxygen is considerably more electronegative than the carbon, so the oxygen has a higher electron density than the carbon and is weakly positive. You could say it’s δ+ C=O δ-.
The key difference between an aldehyde and a ketone is exactly what is bound to the C=O group. If the C=O is sandwiched between carbon atoms, it is a ketone. If the C=O has at least 1 hydrogen bound to it, it’s an aldehyde.
You should know that alkyl groups (a term which usually refers to the rest of the carbon chain) are electron-donating. The more of them on the carbon in the C=O, the less positive the carbon is.
This therefore makes the carbon in the C=O in ketones less attractive to nucleophiles that may form a new bond with it, compared to aldehydes.