The accountant's letter usually expresses a "clean" opinion, which means the accountant or accounting firm believes the financial statements are accurate and that they fairly present the company's financial condition. A "qualified" opinion indicates deficiencies in the company's procedures or presentation (meaning the financial statements may not be accurate or may not conform to GAAP).
An "adverse" opinion, which indicates that a company's financials are misrepresented, is yet another possibility. The most well known opinion is the "going concern", which means that the accounting firm has doubts about the company's financial health and its ability to remain in business.