"Can" may be used to express both ability and possibility.
Ability: "I can play the guitar."
Possibility: "Learning a language can be difficult"
"Could" is the past conjugation of "can". It also may be used to express ability and possibility.
Ability: "When I was a child, I could speak Japanese"
Possibility "I could try to study English"
In your sentence, the speaker is referring to the present, so he will use "can".
If he was speaking about the past, "could" would be correct:
"Yesterday, I couldn't eat till Dad got home."
That are maaaaaany usages of "can" and "could", but I hope this helped you understand it a little better.