Obi-Wan Kenobi was Qui-Gon Jinn's Padawan, but according to The Empire Strikes Back, he was also trained by Yoda at some point in the Star Wars saga. Becoming a Jedi is a rare thing in Star Wars' galaxy, and even those who are Force-sensitive have to undergo rigorous training throughout their entire childhood and most of their young adulthood before becoming a Jedi Knight. From there, they could go into a variety of fields - one of which leads to them being Jedi Masters who sit on the Jedi Council.
When audiences are first introduced to Obi-Wan - chronologically, that is - he's Qui-Gon's apprentice in Star Wars: Episode I - The Phantom Menace. But after his Master dies at the hands of the Sith Lord Darth Maul, Obi-Wan takes it upon himself to take Anakin Skywalker as his apprentice. Shortly thereafter, Obi-Wan is promoted to the rank of Jedi Knight, after first passing the Jedi Trials, thereby ending his training. So when exactly was Obi-Wan trained by Yoda? The answer lies in Star Wars: Episode II - Attack of the Clones as well as extended canon material.
Young children brought into the Jedi Order begin as initiates, sometimes called younglings, which involves going through introductory training and foundational trials before being chosen as a Padawan by an elder. During his lengthy time in the Order, Master Yoda trained all younglings in this initial stage, his instruction being an invariable part of their early education. This can be seen when Obi-Wan visits one of his classes in Attack of the Clones, during which he appears to be well-liked by his young pupils. Although it is more collective lecturing than personal training, this is the time period to which Obi-Wan is referring in The Empire Strikes Back.
While Jedi can take more than one (non-simultaneous) Padawans in their career, and indeed Yoda had many before he took on Count Dooku as his final apprentice, instructing all younglings was a much greater responsibility than any entrusted to most Jedi, and it speaks to the respect that the Jedi Order had for Master Yoda that he would be the one to do so consistently. Seemingly the wisdom gained from living for centuries and being one of the Order's eldest members did not impede his ability to relate to young minds.
As befits a series of films covering three generations with thousands of years of backstory, some of the lore of Star Wars gets to be a bit dense. The history of Jedi apprenticeship in particular feels borderline Old Testament, with Yoda training Dooku, who trained Qui-Gon, who trained Obi-Wan, who trained Anakin. Having so much early influence on many Jedi initiates, Yoda can certainly be found in many such pedagogical lineages, which is potentially an even greater testament to his power than his actual abilities with the Force, having left a legacy of learning that would be essential to preserving the ways of the Jedi into the era of the New Republic and beyond.