What does president Roosevelt's joke mean at the beginning of his speech The speech called Address at the laying of the cornerstone of Franklin D. Roosevelt Library you can Google search the speech?

Answer 1

Here's the first paragraph of the speech.

Half a century ago a small boy took especial delight in climbing an old tree, now unhappily gone, to pick and eat ripe sickle Pears. That was about one hundred feet to the west of where I am standing now. And just to the north he used to lie flat between the strawberry rows and eat sun-warmed strawberries-the best in the world. In the spring of the year, in hip rubber boots, he sailed his first toy boats in the surface water formed by the melting snow. In the summer with his dogs he dug into woodchuck holes in this same field, and some of you are standing on top of those holes at this minute. (Laughter) Indeed, the descendants of those same woodchucks still inhabit this field and I hope that, under the auspices of the National Archivist, they will continue to do so for all.

His comment about woodchucks caused laughter, it's not so much a joke, but merely a statement about woodchuck burrows.