Another question on the site has the following title: Have their been any personal attacks on Sondoland after his change in testimony on the quid-pro-quo? This is not the only time I've seen this term quid-pro-quo in connection with the Trump investigations. What does the term mean, and how does it connect to the ongoing investigations?
To quote Wikipedia on the definition:
Quid pro quo ("something for something" in Latin) is a Latin phrase used in English to mean an exchange of goods or services, in which one transfer is contingent upon the other; "a favour for a favour". Phrases with similar meanings include: "give and take", "tit for tat", and "you scratch my back, and I'll scratch yours" and "one hand washes the other". [...]
Quid pro quo would go on to be used, by English speakers in legal and diplomatic contexts, as an exchange of equally valued goods or services and continues to be today.
Now such exchanges (of "favors") are practiced in diplomacy all the time between countries. The issue that precipitated the Trump impeachment is whether Trump's administration conditioned some Congress-approved aid to Ukraine on Ukraine investigating the Bidens (allegedly for corruption) for their/Hunter's participation in Burisma, a Ukrainian energy company.
The Democrats (and some State Department diplomats) argue that what Trump's envoys (e.g. Giuliani) were requesting was not something that was of value to the US as a whole (i.e. not an obvious "national interest"), but rather that it was mainly in Trump's personal interest, in view of the 2020 election, to have a potential rival (Biden) investigated.
In contrast, the Trump side has both denied that there was a connection between the two issues (aid and the request to investigate) and also denied that the investigation request was anything more than the usual kind of pressure that the US puts on foreign partners to fight against corruption.