When G-d created Adam, the Torah writes: "He blew in his [Adam's] nostrils the soul of life, and man became a living being" (Bereshith:2:7). Onqelos explains that the phrase "a living being" means that G-d gave man the ability to speak. Speech is a gift which differentiates human beings from all other creatures in the world.
Spoken words are extremely valuable. One cannot say - "There's no significance to a word, so don't hold me to it." Our words are the expression of our soul and our thoughts, and usually tell us about the character and personality of the one who spoke them.
The Gemara of Pesahim (3b) relates a story about three Kohanim (priests) who were discussing the show bread they had eaten. One of them said that his show bread was the size of a bean. The other one compared his portion to the size of an olive. The third one made a comparison between the size of his portion and "the tail of a lizard". What was the outcome of the discussion?
Because of the improper use of his language, "the tail of a lizard", which was considered to be an unrefined expression, the need was aroused to investigate his genealogy. As a result, the Kohen was found to be unfit for service in the Temple.
We might not feel that we are on the same spiritual level as the Kohanim, nevertheless, the words that we use reveal our true inner personality and tell the world who we really are. Words are very powerful, although they do not cost anything.