William Shakespeare's Sonnet 55 explores the immortality of the subjects of poetry through the power of verse. Shakespeare uses precise diction to paint a destructive image of time. Phrases like "besmeared with sluttish time" suggest that time has the power to rob subjects of all recognition in the future. Further, the mythological allusion to Mars the Greek god of war serves to strengthen the image of time as a brutal entity. The poem, however, says that the subjects of poems (including this one) will live on in spite of time because the published word is everlasting as it is handed on to generation after generation. Therefore, the theme of this sonnet is about the immortality of the subjects of poetry.
Note the use of alliteration in the third line, with the two words "shall shine." This is something that is used in various places in this sonnet, for example in "wasteful war." You might like to find other examples of alliteration in this sonnet, as it is one of the easiest literary terms to discover.
You also might like to consider the following example of imagery that describes the tombs that will be outlived by the sonnet the speaker writes:
But you shall shine more bright in these contents
Than unswept stone besmear'd with sluttish time.
There is a powerful image in the way in which "time" is personified as a character who is "sluttish" and "besmears" the tombs around her with her mess. The description helps to build up a powerful contrast with the person this sonnet is addressed to, who will "shine" compared to the dull and dirty tombs that time sullies.
You might like to try and analyse the poem by yourself now, and see if you can find any other examples of literary terms. I hope that now I have identified some for you as examples, you can do this for yourself. Good luck!