While it is not possible to share exact hardware requirements for the software as performance of the application depends heavily on the type of content and features that are used within a World, it is possible to provide some recommendations and basic requirements for a bare-minimum usage.

Client(player or GM) Minimum Requirements

Relatively modern computer running Windows 10, MacOS, or Linux operating systems with support for 64-bit architecture.
An integrated GPU to enable hardware acceleration.
8GB of RAM
A monitor no smaller than 1366x768. At this minimum resolution many aspects of the UI will feel cramped.
A modern web browser like Chrome, Firefox, Opera, or Edge with hardware acceleration enabled. (Safari is not a supported browser at this time).

Client(player or GM) Recommended Specifications

Relatively modern computer running Windows 10, MacOS, or Linux operating systems with support for 64-bit architecture.
A dedicated GPU which supports WebGL 2.0.
16GB of RAM
A monitor with 1920x1080 or higher resolution.
A mouse. You can use the software with a touchpad but the current software is designed for mouse and keyboard.
Chrome or a Chromium-based browser provides an experience closest to the FVTT desktop application.

Hosting Requirements

There are three main modes to host a Foundry server: Self-Hosted, Cloud Hosted, and Partner Hosted. Partner Hosted offloads the responsibility of server hardware to the hosting partner, so it needs no list of requirements here.

Self-hosted Minimum Requirements

Everything listed for client minimum requirements above, and:

An internet service provider which allows port-forwarding via IPv4, and which does not use Carrier Grade Network Address Translation (CG NAT).
A router configured to port-forward incoming connections to your PC.
Your operating system's firewall configured to allow connections on the port FVTT is using.
An internet upload speed of at least 5MB/s (40mbps) is recommended since the host needs to transfer image, video, or audio assets to connected players. For hosts with slow upload speeds, serving your media files from a cloud storage location (for example: using S3 File Storage Integration) will avoid this limitation.

note: If your ISP provides an IPv6 address and your players can connect via IPv6 (confirmed via this link) there is a high possibility you do not need to set-up port forwarding, and your players will be able to connect to you via IPv6 instead, negating the need for Port-Forwarding.

Dedicated Server Minimum Requirements

at least 1 vCPU (2 recommended)
At least 1GB storage space.
1 GB RAM
Firewall and security settings (dependant on host) configured to allow connections on the port required.