In plain words
It is very dangerous. Each sense has only one power. But this tongue has two kinds of power. One is speaking, the second is tasting. That is why Jayadeva,
Jihverasajñe madhurapriyatvaṃ satyaṃ hitaṃ tvāṃ paramaṃ vadāmi āvaraṇamekā madhurākṣarāṇi govinda dāmodara mādhaveti,
went on describing it in many ways. O tongue that knows taste! O tongue that speaks sweet words! O honoured tongue! On you I will use nothing but the words of God. I do not wish to make such a tongue impure. It must be used for Satya alone. I must use it for what is pleasing. In just the same way, in the Bhagavad Gītā, ‘Anudvegakaraṃ vākyaṃ satyaṃ priya hitaṃca’, they said. This Satya too must not cause agitation. It is endowed with every kind of sacred quality. That is why we must use the tongue with the greatest care. It is through this very tongue that bondage and Mokṣa come to us. What gives lasting happiness and lasting Ānanda is the tongue itself. Of all the senses this tongue is the most important and the most dangerous. That is why these senses must be used with care.
For the use of such a sense, the ancient ṛṣis followed many kinds of discipline. The tongue must not be given everything it asks for. If you keep giving the tongue all it asks for, in the end it is ready to swallow you yourself. That is why, to restrain these tastes, the ancient ṛṣis followed certain disciplines. They called them the Cāturmāsya vows. This Cāturmāsya vow came into being with the aim of training the tongue rightly for four months, not giving it all it asks for, and holding it on the right path. From the coming month of Āṣāḍha, they will keep this vow for four months. There is another reason for it as well. Since this is the rainy season, it is very hard for sannyāsīs to travel from one place to another. With the rains, the rivers may be in flood. The streams may be running. Travel is hindered. So they began this Cāturmāsya vow so that, staying in one place, they might make these four months fruitful through the contemplation of God. They would go to the forest, build a hut there and do tapas inside the hut. There no tasty food was to be had, only roots and tubers.