What is meant in line 8 in Sonnet 18, which is "By chance or nature's changing course untrimm'd?"

You need to read the line in context and not separate from the rest of the poem. That said, here is the full text of the sonnet:

Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd,
And every fair from fair sometime declines,
By chance, or nature's changing course untrimm'd:
But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st,
Nor shall death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st,
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.

Shakespeare is comparing his love to a summer's day, and she is much lovelier. The flowers that bloom in May often get blown away by the winds, or sometimes the sun is too hot or clouded over.

Then we come to the line you're asking about: "And every fair from fair declines,/ By chance, or nature's changing course untrimm'd." Everything that is "fair" (beautiful) about summer eventually "declines," either by chance (somebody comes along and plucks it up; insects destroy it; etc.) or by the change of season (nature's changing course).

Visit the link below for a critical analysis of this poem.