If you have to ask, you've been hiding under a rock or in complete denial. There are many. The two the House chose to act on are obstruction of Congress and abuse of power.
Re: Obstruction
Trump is well within his rights to not testify and not allow witnesses.
The law states, in such cases, that it be taken to the courts.
The Democrats refused, since it would "take too long and this was urgent".
Now, they're holding up sending to the Senate.
So, they bypassed the system claiming urgency, now they are delaying.
Or, he's impeached on an obstruction charge on a weak, partisan impeachment, that had to change the charges to a vague terminology to get it passed.
If you are really concerned about the US, what aren't you concerned that in 2016:
1. Democrats PAID for a fake dossier, and the FBI altered documents.
2. This led to illegal spying on a campaign
3. This lead to an investigation that cost millions, and likely influenced the 2018 mid terms.
All proven stuff.
Not like the heresay and presumption that led to this phony impeachment.