While two of the other answers are good answers and generally provides information on who Lor San Tekka is or The San Tekka's are, it doesn't get into more specifics of their signifigance to either Luke Skywalker or The High Republic and/or the connections that are being made between them.
The Rise of Kylo Ren miniseries comic for instance, features one story of how Ben Solo was introduced to an iteration of the Knights of Ren. He is traveling with Luke Skywalker and Lor San Tekka at a time were Luke Skywalker is out searching information about the past Jedi and/or The Force. They come upon a temple from The High Republic Era filled with relics, but are interceded by The Knights of Ren and they do battle...
The comic also makes other connections to the High Republic such as Ben Solo finding a holocron from Avar Kriss, a prominate Jedi during the era. But even more interesting is that [one of the] Snoke(s) is operating out of Amaxine space station featured in the novel Into The Dark, which is about the Jedi investagting the station and accidentally releasing a monsterious Force Sensative plant species called The Drengir across the galaxy that was previously sealed away on the station by The Sith!
Again as mentioned in one other answer, these connections to the High Republuc in part serve, because The San Tekka Clan or Family is another feature of the ongoing story in the High Republic Era, but one San Tekka in particular, Mari San Tekka, is VITAL to the early plights of the story's main antagonist Marchion Ro, as many San Tekkas have a unique Force ability that Ro hopes to use to cause chaos, but also aid him against his destruction of the Republic and The Jedi.
And while it is not entirely clear why The High Republic keeps connecting to this era between Return of the Jedi & the Sequel Trilogy, but more specifically Luke Skywalker (in Star Wars ongoing comic current set between The Empire Strikes Back & Return of the Jedi Luke comes across a force vision of Elzar Mann also), setting aside: just for continuity's sake,we can at least surmise that The High Republic, especially in terms of the Jedi informs the Skywalker Saga, and Lor San Tekka has become a curious link to the Jedi's pre-Skywalker Saga's history along with his connection to Luke & Ben.