The Radak says that in context this pasuk is talking about the Jewish redemption from exile:. “For with joy you shall go forth from exile and with peace you shall be brought to your land. The mountains, hills and fields are a parable for the whole world rejoicing. They will be clapping hands in joy proclaiming ‘Long lives the king!’… Another interpretation is that mountains and hills refer to the leaders and the trees in the field refer to the commoners.” (Click here and here for Hebrew translation.)
At the end of the Haftorah, there is a famous pasuk that we say in our Slichos before Sh’ma Kolienu. Yeshayahu prophesizes in the name of Hashem: “I will bring them to My holy mountain, and I will cause them to rejoice in My house of prayer, their burnt offerings and their sacrifices shall be acceptable upon My altar, for My house shall be called a house of prayer for all people” (Yeshayahu 56:7).
The Radak explains that after the Beis Hamikdash will be rebuilt speedily in our days, just as a guest enters someone’s house and the hosts accepts him with happiness and joy, so to I, Hashem, will command the kohanim to accepts anyone who comes to convert with happiness and joy. “And I will cause them to rejoice in My house of prayer,” meaning they, the non-Jews coming to convert and certainly the Jews who left their religion and are now coming back , will be happy when they see themselves come year after year to the Azara, which is the house of prayer, with the rest of the Jewish people bringing their prayers, burnt offerings, and other sacrifices. (Click here and here for Hebrew translation.)
In the advent of Moshiach the entire world will recognize that Hashem is One, the Creator and Sustainer of everything, All Powerful, and King of All Kings, and that the Jewish people are the chosen holy nation, priests and light unto the world. Why then does it seem that Hashem has to instruct the kohanim to accept anyone wishing to convert with open hands and a smile, just like a host accepting his guests? Shouldn’t they naturally be overwhelmed with joy that everyone is coming together and wants to be closer to Hashem? The whole world will be jumping with joy and singing “long lives the King,” as we saw above; why then would the kohanim need instructions to accept converts with happiness and joy? Furthermore, once the converts and even the baalei teshuva are in the Beis Hamikdash, mixing with the crowd to pray and give offerings to Hashem, why then does Hashem have to “cause them to rejoice in My house of prayer” as if they would feel uncomfortable otherwise?
The lesson is that people will still be human even when Moshiach reveals himself. It would seem that even at a joyous time like this when everyone comes together in unity, and the past philosophies and history are left behind, still in all there is a natural tendency for the leaders and more experienced to be skeptical of the newcomers. That is why Hashem must command the kohanim to treat those that want to convert as a host would happily treat his guests. This is also why the converts and those Jews that return back to their roots might seem to feel uncomfortable when walking into such an experience, since there is a natural tendency to feel the jitters and out of place. Therefore Hashem will be right next to them, bringing them to His holy mountain, “and will cause them to rejoice in His house of prayer” just as everyone else in ecstasy will come to pray and show their gratitude and offerings to Hashem, may this moment come speedily in our days.