This is the 14349th most frequent Spanish word.
viajarán
"Viajarán" means "They will travel" in English.
Here, 'viajarán' is used in its simple future tense form to indicate an action (traveling) that will happen in the future.
Ellos viajarán a España el próximo verano.
They will travel to Spain next summer.
In this case, 'viajarán' implies a hypothetical future action subject to the condition of having free time, although in formal language 'viajarían' might be used for hypothesis.
Pienso que viajarán mucho si tienen más tiempo libre.
I think they would travel a lot if they have more free time.
Here, 'viajarán' asks about the time of the planned future action of traveling, directed towards the second person plural form 'ustedes'.