Dropbox uses TLSv1.2 with 128 bit AES encryption when transferring data between your computer and the Dropbox servers, and the data is encrypted with 256 bit AES when stored on their servers.
One thing to note is that when using the Dropbox client, your Dropbox data appears to your operating system as just another folder, indistinguishable from, e.g. My Documents or Desktop This means it is susceptible to anything that modifies data on your local computer. For example, malware such as CryptoLocker could affect your Dropbox folder just as it affects any other data stored on your computer. Similarly, if someone gains access to your computer, they will likely be able to access your Dropbox folder.
When using the Dropbox client, it is particularly important to ensure that you have Sophos anti-virus and CrowdStrike installed. That your operating system is configured to take automatic updates and that you are using Strong Passwords on your computer.
What sort of things should I not store in the cloud?
Privacy of Electronic Communications, Electronic Files, and Other Files
Dropbox for Business Security Whitepaper
Dropbox for Business Information Security Architecture